1. Field of the Invention
This invention relates to a hybrid vehicle, and more particularly, to a hybrid vehicle that has the function of running while making a changeover between so-called series hybrid running and so-called parallel hybrid running.
2. Description of Related Art
A hybrid vehicle that runs while making a changeover between series hybrid running (hereinafter referred to also as “series HV running”) in which a rotating electrical machine generates a running driving force through the use of an electric power that is generated by a motive power of an internal combustion engine and parallel hybrid running (hereinafter referred to also as “parallel HV running”) in which the internal combustion engine generates a running driving force and the rotating electrical machine assists the running driving force according to need is disclosed in, for example, WO 2011/129196.
In the hybrid vehicle described in WO 2011/129196, the changeover condition is changed based on the state of charge (the SOC) of a battery in the control for making a changeover between series HV running and parallel HV running in accordance with the vehicle speed. Thus, running control can be stabilized and facilitated.
In the hybrid vehicle as described above, during series HV running, the internal combustion engine is mechanically decoupled from driving wheels, so the rotational speed of the internal combustion engine can be set independently of the current vehicle speed (the rotational speed of the driving wheels). Thus, the degree of freedom in setting an operating point that is defined as a combination of a rotational speed and a torque of the internal combustion engine is enhanced, so the internal combustion engine can be operated with the operating point corresponding to high efficiency selected.
On the other hand, during parallel HV running, the internal combustion engine is mechanically coupled to the driving wheels, so the rotational speed of the internal combustion engine is restricted according to the vehicle speed. Therefore, in order for the internal combustion engine to operate at an operating point with high thermal efficiency, the output torque of the internal combustion engine needs to be increased and reduced with respect to the torque needed for running. For example, in the case where the operating point of the internal combustion engine is set in a high-efficiency region by increasing the torque of the internal combustion engine, a surplus output of the internal combustion engine is used to generate an electric power by the rotating electrical machine, and is thereby absorbed as a charging electric power of an electrical storage device.